Method of making corrosion resistant metallic plates particularly useful as lithographic plates and the like

ABSTRACT

In a process for making lithographic plates which includes a step for forming on metallic plates a protective layer or film which additionally provides a hydrophilic film on at least one surface thereof and in the course of which the metallic plates are placed in contact with water at diverse steps of the process, the improvement consisting in using purified water. The metallic plates initially take preferably the form of a continuous web of an appropriate metal, such as aluminum, which is successively: cleaned by suitable detergent, rinsed in purified water, provided with a protective layer or film, rinsed again in purified water, and coated with a sensitizing diazo resin or the like. The metallic plates are provided with a protective layer or film by dipping in a solution of a soluble silicate in purified water at a predetermined temperature, or alternately, the metallic plates may have a protective layer electrolytically formed thereon, the electrolyte consisting of a solution of sodium silicate in purified water. The use of purified water, which is basically tap water which has been de-ionized, de-chlorinated, de-gasified, and which is free of solid particles, in the diverse steps of the process prevents the formation of &#39;&#39;&#39;&#39;black spots&#39;&#39;&#39;&#39; on the surface of the plates provided with a photosensitive coating of diazo resin or the like.

United States Patent 9| Casson, J r.

| lMarch 13, 1973 I 1 METHOD OF MAKING CORROSION RESISTANT METALLICPLATES PARTICULARLY USEFUL AS LITIIOGRAPI-IIC PLATES AND THE LIKE doned.

[52] U.S. Cl. ..10l/463, l01/426, 210/39 [51] Int. Cl ..B41n 3/00, BOld15/00 [58] Field of Search ..l01/463; 210/39 [56] References CitedUNITED STATES PATENTS 543,638 7/1895 Bea] ..l0l/463 X 3,350,206 10/1967Leonard ...10l/463 X 3,549,365 12/1970 Thomas ...l01/463 X 2,520,1898/1950 Zarow ..2l0/39 X 3,408,289 10/1968 Gustafson.... .....2l0/39 X3,444,079 5/1969 Bowers ..210/39 X Primary Examiner-Clyde I. CoughenourAttorneyRobert C. Hauke et a1.

[57] ABSTRACT In a process for making lithographic plates which includesa step for forming on metallic plates a protective layer or film whichadditionally provides a hydrophilic film on at least one surface thereofand in the course of which the metallic plates are placed in contactwith water at diverse steps of the process, the improvement consistingin using purified water. The metallic plates initially take preferablythe form of a continuous web of an appropriate metal, such as aluminum,which is successively: cleaned by suitable detergent, rinsed in purifiedwater, provided with a protective layer or film, rinsed again inpurified water, and coated with a sensitizing diazo resin or the like.The metallic plates are provided with a protective layer or film bydipping in a solution of a soluble silicate in purified water at apredetermined temperature, or alternately, the metallic plates may havea protective layer electrolytically formed thereon, the electrolyteconsisting of a solution of sodium silicate in purified water. The useof purified water, which is basically tap water which has beende-ionized, dechlorinated, de-gasified, and which is free of solidparticles, in the diverse steps of the process prevents the formationofblack spots on the surface of the plates provided with aphotosensitive coating of diazo resin or the like.

1 Claim, 2 Drawing Figures METHOD OF MAKING CORROSION RESISTANT METALLICPLATES PARTICULARLY USEFUL AS LITIIOGRAPIIIC PLATES AND THE LIKE This isa continuation of application Ser. No. 5,531, filed Jan.26, 1970, nowabandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention belongs generally to the field of methods and processes formanufacturing lithographic plates, and more particularly the presentinvention relates to the use of purified water in such methods andprocesses with the accompanying result of providing lithographic platesentirely devoid of the black spots which have heretofore plagued thelithographic art and presented a problem preventing the generalacceptance of pre-sensitized lithographic plates.

2. Description of the Prior Art Pre-sensitized lithographic platescurrently in use today generally include a metallic support memberhaving, for example, aluminum as its principal component, a surface ofwhich has been treated by chemical or electrolytical methods to providea barrier layer or film which prevents interreaction between thephoto-sensitive diazonium salts, or other photo-sensitive ornon-photo-sensitive coatings, placed upon such surface of the supportmember. The barrier layer formed on a surface of the metallic memberprovides a chemical pacification which increases the shelf life of thelithographic plate, facilitates the processing of the plate afterexposure and improves the length of the printing run and the quality ofthe print. The barrier layer is obtained according to the prior art bysubjecting the metallic surface to the action of an aqueous solution ofone or several of a plurality of compounds, examples of which includehydrolized cellulose ester, sodium phosphate glass, alkali metalsilicates, sodium metaborate, phosphomolybdate, sodium silicate,silicomolybdate, water-soluble alkylated methylomelamine formaldehyde,polyalkylenepolyaminemelamine-formaldehyde resins, urea-formaldehyderesin plus polyamide, polyacrylic acid, polynethacrylic acid, sodiumsalts of carboxymethylcellulose, carboxymethylhydroxyethil-cellulose,zirconium hexafluoride, etc.

A solution which is often used in the prior art methods is an aqueoussolution of sodium silicate in which the metallic plate, forming thelithographic plate support member, is dipped, or which is applied to thesurface of the plate. The solution is preferably heated before dippingthe plate therein or before applying to the surface of the plate, andthe plate surface is optionally washed with an acidic medium in order toharden the silicated surface and neutralize any alkali that remains onthe surface. Another method of providing a protective barrier isdisclosed in the copending application entitled Corrosion ResistantMetallic Plate Particularly Useful As Support Members forPhotolithographic Plates and the Like," Ser. No. 811,267, filed Jan. 21,1969, and assigned to the Assignee of the present application. Theprocess disclosed in the aforementioned patent application provides anelectrolytic process for forming on the surface of a metallic plate apacified corrosive resistant hydrophylic surface layer. The processconsists in electrolytically forming on the metallic plate theprotective layer or film by utilizing an electrolyte consisting of anaqueous solution of preferably sodium silicate, the metallic plateconstituting the anode in the process.

In addition to acting as a barrier layer between the metal of themetallic plate and the diazo resin, the silicated surface forms ahydrophilic surface which partially acts as an initial water-carryingsurface when the processed plate is placed in a printing press. Thehydrophilic surface thus formed is desirably relatively insoluble in thefountain solution used in a printing press in order to preventunder-cutting the hydration of the image areas.

The formation of a barrier layer thus requires placing the metallicplates in contact with water, and, additionally, several other steps inthe lithographic plate manufacturing process also necessitate placingthe metallic plates in contact with water.

In prior art manufacture, sheets of the metallic plate which generallyconsist of aluminum are first cleaned with a suitable detergent andrinsed with tap water, grained with a slurry of sand and tap water andagain rinsed with tap water prior to immersion in the sodium silicatesolution, or prior to immersion in the electrolyte. The silicated platesare again rinsed in tap water prior to the coating of a photosensitivematerial such as diazonium salts. Since the surface of the metalcontains some materials which are not rinsed off as well as materialsplaced thereon by the tap water during the rinsing and silication steps,some of which may be in ionic form and some in particle form, thesilicate solution, being alkaline, precipitates many of the ionicimpurities and deposits them on the metallic plates. There is generallyno scrubbing action in the silicate tank irrespective of which processis utilized, and therefore a tendency exists for the impuritiesdeposited on the plates to remain thereon. The bulk of the impuritieswhich remain on the metallic plates comes from the tap water andgenerally takes the form of chlorides or chlorine from the water supplyitself and copper or iron or zinc from the water supply piping. Theplacing of these impurities on the plate surface during the washing,graining and rinsing steps with tap water and the subsequentprecipitation of the impurities on some areas of the plates upon entryinto the silication tank prevent proper passivation of the platemetallic surface in those areas. The presence of such impurities resultsin undesirable reactions specially on an aluminum surface. Copper, forexample, would be reduced to the free state by aluminum and the tendencywould be for this reaction to continue and act as a corrosion center.Such a corrosion center would react quite differently from the sodiumsilicate passivated area surrounding it, and would result in what isknown in the lithographic industry as black spots.

It is therefore desirable to provide a manufacturing process forpre-sensitized lithographic plates in which metallic plates may bewashed, rinsed, grained and provided with a protective layer as in thehereinbefore described processes, yet one which completely eliminatesblack spots."

SUMMARY OF THE INVENTION The present invention, which will be describedsubsequently in greater detail, provides a method for manufacturinglithographic plate including forming on the surface of metallic plates,such as generally used as support members for a coating of diazoniumsalts or the like, a pacified corrosive resistant hydrophilic surfacelayer greatly enhancing the lithographic and printing performance, andin particular the present invention provides a method of making alithographic plate of the type hereinbefore described which completelyeliminates the formation ofblack spots.

According to the present invention, the formation of black spots" iscompletely eliminated by the use of purified water in each of thediversed steps of lithographic plate manufacture. The purified water isbasically a de-contaminated, de-ionized, de-gasified and de-chlorinatedwater. Tap water is purified by filtering through an activated carbonfilter which removes from the water trapped gasses, such as freechlorine and oxygen, and any solid particles. The water then enters asecondary filter filtration system in which the use of iron, copper orother objectionable metal, is carefully avoided in vessels or piping.The preferred system carries the filtered water in polyethylene,polyvinyl chloride, glass, or stainless steel piping and utilizesvessels made of the same materials. The water leaving the carbonfiltration system enters a strong cationic resin bed, which removes thepositive ions, such as copper, iron, and magnesium, and replaces themwith a hydrogen ion (H+). The water leaving the cationic resin bed issightly acidic and is passed through a second bed containing a stronganionic resin, which replaces the nitrate, phosphate, and sulphate,which are in the form of an acid, with a hydroxyl ion (OH). Suchcationic and anionic resin beds are available from severalmanufacturers. The resultant water leaving the second resin bed issubstantially pure water, and is utilized in each of the several stepsin manufacturing lithographic plates which requires placing the platesin contact with water.

The use of purified water in the several steps of the process of makinglithographic plates and the avoidance of copper, iron or zinc piping andvessels in the water system result in obtaining lithographic platesentirely devoid of the black spots spoiling lithographic platesmanufactured by conventional methods.

Other advantages, applications and objects of the present invention willbecome apparent to those skilled in the art when the accompanyingdescription of one example of the best mode contemplated for practicingthe present invention is read in conjunction with the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic representation,in the form of a block diagram, of an example of arrangement of elementsfor the purifying of water according to the method ofthe presentinvention; and

FIG. 2 is a flow chart representation of one process of makinglithographic plates in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present inventioncontemplates purifying tap water by means of a purification system,generally indicated by the numeral at FIG. 1. The purification system 10consists of an activated carbon filter 12, a

strong cationic resin bed 14, and a strong anionic resin bed 16, throughwhich the tap water is caused to circulate. The piping 18 carrying thewater between the cationic resin bed 14 and the anionic resin bed 16,the outlet piping therefrom and the vessel containing the beds 14 and 16are preferably made of stainless steel. Other inert materials may,however, be utilized, such as polyvinyl chloride, polyethylene, orglass. The purpose of using such materials for the piping and vessels isto eliminate contamination of the water by iron, copper or other metal.

The tap water enters the activated carbon filter 12 and gasses such asfree chlorine and oxygen trapped in the water are removed. Solidparticles in suspension in the water are also removed in the carbonfilter 12. The water leaves the carbon filter and enters the strongcationic resin bed 14, which removes the positive metal ions, such ascopper, iron and magnesium ions, and replaces them with a hydrogen ion(H+). The water leaving the strong cationic resin bed 14 is then causedto flow through the strong anionic bed 16. The water leaving thecationic resin bed 14 is slightly acid and contains small quantities ofnitric acid, phosphoric acid and sulfuric acid. Upon entering andfiltering through the anionic resin bed 16, the nitrates, phosphates andsulphates in the form of an acid are replaced by a hydroxyl ion (OH).The resultant fluid leaving the anionic resin bed 16 is pure water, freeof solid particles and gases and free of contaminating elements such aschloride, chlorine, nitrate, copper, iron, magnesium and the like.

Referring now to FIG. 2, there is schematically shown a flow chart ofthe principal consecutive steps in the manufacturing of pre-sensitizedlithographic plates. A continuous web of metal 20, preferably aluminum,is uncoiled by appropriate mechanisms and is continuously fed throughseveral work stations, each of which is adapted to accomplish one of theparticular steps required for manufacturing lithographic plates. Themetal strip or web is first washed, as shown at 22, to remove oil andother contaminants from the surface of the metal. The washing may beeffected, for example, with water and detergents. The continuous web ofmetal 20 is subsequently rinsed with water as shown at 24. Graining step26 is representative of a graining operation in which the surface of themetal web is grained, or roughened, by being impacted with a slurryconsisting of sand or other abrasive particles, in suspension in water.The grained web of metal is then again rinsed as shown at 28, to removethe slurry and any particles of metal removed during the grainingoperation which may adher to the web surface. The web of metal is thenprovided with a superficial barrier layer, as shown at 30, for example,by any of the silication methods hereinbefore mentioned, again rinsed asshown at 32 and coated as shown at 34 with a photosensitive materialsuch as a diazo resin or the like according to the conventional methodcommonly utilized in the continuous line lithographic platemanufacturing industry.

It is thus obvious that the metallic web is almost constantly in contactwith water in the course of the manufacturing process schematicallyrepresented at FIG. 2. Water is also used for the preparation of theaqueous bath for silication or other treatment. Prior art manufacturingprocesses for lithographic plates utilize tap water" as supplied fromthe water mains. The present invention results from the discovery thatthe so-called black spots appearing on pre-sensitized lithographicplates manufactured by prior art processes and which, in turn, causedefective or smudged prints result from the use of ordinary tap water inthe manufacturing process thereof. The present invention, consequently,contemplates purifying the ordinary tap water, from commonly availablewater supplies, on a continuous basis in an economical way, forutilization at each step of lithographic plate manufacturing processesin the course of which a metallic support member, usually made ofaluminum, is placed in contact with water or an appropriate aqueoussolution. it has been discovered that black spots are eliminated fromfinished lithographic plates by utilizing water purified in the mannerhereinbefore described, and by utilizing piping and vessels made ofappropriate inert materials for containing such water at each step ofthe lithographic plate manufacturing process, such inert materialsconsisting preferably of stainless steel, polyvinyl chloride,polyethylene, or glass, thus avoiding recontamination of the water afterit has been purified.

EXAMPLE 1 A continuous web of l 100 aluminum 29 1% inch wide and 0.009inch thick was passed through a washing station where it was scrubbedwith water and detergents to remove oil, dirt and other contaminationfrom the surface thereof, and rinsed in purified water. The surface ofthe continuous web of aluminum was grained at a line speed of 12 feetper minute using a sand slurry with the purified water. After rinsing,the web was electrolytically silicated according to the processdescribed in detail in copending application Ser. No. 811,267, bypassing through an appropriate electrolyte bath at a predetermineddistance from an electrode, consisting of a stainless steel grid, thegrained surface of the web being disposed opposite the electrode grid.The metallic web was connected to the positive terminal of a DC powersupply and the stainless steel electrode grid was connected to thenegative terminal of the power supply such that the metallic web waselectrolytically anodic and the stainless steel grid waselectrolytically cathodic. The spacing between the web and the cathodewas 4 inches, and the cathode extended [0 feet along the length of theweb. The electrolyte solution consisted of solution of purified waterand Star Brand 42' Baume sodium silicate, defined as (1 Na 0:2.5 SiO),and sold by Philadelphia Quartz Company, the concentration of silicatein the solution being equivalent to 3.10 percent of sodium silicate byweight having a pH of approximately 13. A voltage of 30 volts wasapplied across the web and the cathode.

After silication, the web was rinsed in purified water and the silicatedsurface of the web was coated with a conventional diazo resin, accordingto the conventional methods used in the lithographic plate manufacturingindustry. The diazo resin used for all tests conducted in the severalexamples disclosed herein was Diazo Resin No. 4 manufactured byFairmount Chemical Company. After cutting to length, the sample plateswere exposed and developed by means of a one-step developer, whichdevelops the image at the same time as it lacquers it.

By using only purified water in the diverse washing, rinsing andgraining steps, by utilizing only purified water in the preparation ofthe electrolyte and by avoiding placing the water purified according tothe method hereinbefore described in vessels or pipes made of materialother than an inert material such as stainless steel, polyvinylchloride, polyethylene or glass, the formation of corrosive centersshowing up as black spots" on conventional lithographic plates wasentirely eliminated, even though some of the plates were used aconsiderable time after the plates had been manufactured.

Other salts which may also be included in the electrolyte, in additionto sodium silicate, include potassium and calcium silicates, phosphates,chromates, borates, vanadates and molybdates. These and otherconstituents when used alone, or in combination in electrolytic solutionwith purified water, provide plates exhibiting the same qualities as thesodium silicated plates, in that the resultant lithographic plates arefree of the corrosive centers which result in black spots.

EXAMPLE 2 A web of aluminum similar to that described in example 1 wasprocessed in the same manner as previously explained except that it wassilicated by the more conventional chemical method by being dipped in asolution of sodium silicate maintained at a temperature of the order ofl2 l2 F. This was done by running the web of aluminum foil continuouslythrough an immersion bath of an aqueous solution of approximately 4percent of Star Brand sodium silicate by weight, utilizing waterpurified according to the method hereinbefore described. The web wasimmersed in the bath for about 5 minutes, which requires, at a linespeed of 12 feet per minute, a tank about 60 feet long. After thistreatment, the excess soluble silicate and any other soluble materialspresent were immediately washed away with purified water, leaving aninsoluble surface layer. The silicated surface of the web was coatedwith the conventional diazo resin in the same manner as described inExample 1, and the web was cut to length. The sample plates were exposedand developed by means of the one step developer. The resultantlithographic plates were stored for a period of several days and, uponexamination, the plates indicated no deterioration in the form ofcorrosion centers which result in the aforementioned black spots.

Consequently, lithographic plates manufactured according to the processof Example 1 as well as the method of Example 2, utilizing exclusivelywater purified by the purification method disclosed hereinbefore andavoiding subsequent contamination of the purified water by using onlyinert materials for containing such purified water, present none of thedisadvantages of the lithographic plates of the prior art, speciallywith respect to the formation ofblack spots." When the silicated surfaceof the plates is coated with a diazo resin, or the like, the silicatedlayer provides a good anchoring surface for the diazo resin or otherphotosensitive material, and a generally hydrophilic surfacesubstantially resistant to the attack of fountain solutions when theplate, after processing, is placed in a conventional printing machine.As a result of using purified water at every step of the diverseprocesses placing the metallic plates in contact with water, the workingsurface of the plates is free of chlorine, chloride, copper, iron, zinc,magnesium, and the like, and the present invention thus permits toobtain lithographic plates which are resistant to the formation ofcorrosion centers, such that the working surface thereofis entirelydevoid ofblack spots.

By contrast, lithographic plates manufactured according to the processof Example 1, as well as by the process of Example 2, but utilizingordinary tap water in the steps of the processes wherein the metal foilweb is placed in contact with water or with an aqueous solution ofsilicate or other salt, exhibit numerous black spots in a few days, oreven at the time of processing. It has been discovered that,statistically, the propensity to the formation of such black spots ismuch more pronounced in lithographic plates made according to the methodof Example 2 than in lithographic plates made according to the method ofExample 1. it seems that this is due to the fact that the silicationstep of Example 1 is effected by electrolytic means which tend to removefrom the surface of the metallic foil the free ions capable of reactingwith the foil metal at the surface thereof with the resulting formationof corrosion centers. Consequently, lithographic plates manufacturedaccording to the process of Example I, even utilizing tap water in theprocess, are not as prone to the formation of black spots as arelithographic plates manufactured by the method of Example 2, alsoutilizing tap water at every step of the process wherein the metallicfoil is placed in contact with water. It is nevertheless obvious that inthe manufacture of lithographic plates, the use of water purifiedaccording to the present invention and the avoidance of recontaminationof the purified water also according to the present invention,preferably at each step in the course of which the metallic foil is incontact with water, and, at least at each step of the process prior tothe final rinse of the metallic foil or of the plates, permits to obtainhigh quality lithographic plates presenting none of the inconveniencesof the prior art lithographic plates with respect to black spots whichresult in defective prints.

It will be appreciated that although the present invention has beendescribed in combination with specific examples of manufactures oflithographic plates of the pre-sensitized type, the present invention isequally useful when applied to the manufacture of socalled wipe-on"lithographic plates, i.e., plates having no sensitized coating thereonand which are sensitized by the user just prior to exposure andprocessing for printing use.

Having thus described the present invention, what is sought to beprotected by United States Letters Patent is as follows:

Iclaim:

1. In a process of manufacturing lithographic plates which comprises theconsecutive steps of washing metallic plates with water and detergents,rinsing said metallic plates with water, graining a surface of saidmetallic plates with an abrasive aqueous slurry, rinsing said metallicplates with water, treating said metallic plates by immersion in anaqueous bath ofa compound, rinsing said metallic plates with water andplacing a photosensitive coating on a grained surface of said metallicplates, the improvement consisting in the use of purified water in eachof said washing, gramtng, first rinsing, treating and second rinsingsteps, said purified water being obtained from normally available tapwater by circulating said tap water through an activated carbon filterfor removing solid particles and trapped gas therefrom, through acationic resin bed for removing cations therefrom and replacing saidcations by hydrogen cations, and through an anionic resin bed forremoving anions therefrom and replacing said anions by hydroxyl anions,and in preventing recontamination of said water by utilizing pipes andvessels made of an inert material at every step of the purificationprocess of said water and at every step of said lithographic platemanufacturing process involving the use of water.

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